Professor Craig Simmons

The freshwater resources of coastal aquifers are vulnerable to seawater intrusion (SWI) caused by the current rising trends in sea levels, amongst other factors. Recent studies have examined the extents, rates and timescales associated with SWI induced by expected levels of sea-level rise (SLR), but have neglected the effects of transgression (i.e. inland migration of the shoreline in response to SLR). In this chapter, variable-density numerical modelling is used to examine the implications of transgression for a range of SWI scenarios based on simplified coastal aquifer settings. Vertical intrusion during transgressions can involve density-driven convective processes, causing substantially larger amounts of seawater to enter the aquifer and creating more extensive intrusion than that of horizontal SWI occurring in the absence of transgression. Interestingly, cases of transgression where no vertical mixing occurs involve reduced landward migration of the wedge toe, relative to otherwise similar situations without transgression. The rates and extents of SWI caused by transgression and associated vertical mixing appear to be almost non-responsive to the choice of landward boundary condition, contradictory to the findings of SLR-SWI studies that neglect transgression. The findings of this study suggest that modelling analyses that neglect the effects of transgression in SLR-SWI investigations may underestimate significantly the rates and extent of SWI.

Arid and semi-arid climates are mainly characterised as those areas where precipitation is less (and often considerably less) than potential evapotranspiration. These climate regions are ideal environments for salt to accumulate in natural soil and groundwater settings since evaporation and transpiration essentially remove freshwater from the system, leaving residual salts behind. Similarly, the characteristically low precipitation rates reduce the potential for salt to be diluted by rainfall. Thus arid and semi-arid regions make ideal ¿salt concentrator¿ hydrologic environments. Indeed, salt flats, playas, sabkhas and saline lakes, for example, are ubiquitous features of arid and semi-arid regions throughout the world (Yechieli and Wood,2002). In such settings, variable density flow phenomena are expected to be important, especially where hypersaline brines overlie less dense groundwater at depth. In contrast, seawater intrusion in coastal aquifers is a global phenomenon that is not constrained to only arid and semi-arid regions of the globe and is inherently a variable density flow problem by its very nature. These two examples make it clear that variable density flow problems occur in, but importantly extend beyond, arid and semi-arid regions of the globe. The intention of this chapter is therefore not to limit ourselves to modelling arid zone hydrological systems, but rather to present a more general treatment of variable density groundwater flow and solute transport phenomena and modelling. The concepts presented in this chapter are therefore not climatologically constrained to arid or semi-arid zones of the world, although they do apply equally there.

In fractured rock aquifers, apparent groundwater ages obtained with environmental tracers (e.g.,14C, CFC-12, and 3H) usually do not represent the hydraulic age of the water. Diffusion of solute between the fractures and matrix results in apparent ages that are greater than hydraulic ages, and that may be different for different tracers. We use approximate analytical solutions and numerical simulations of tracer transport through fractured porous media to illustrate the dependence of 14C and CFC-12 ages and 3H concentrations on fracture and matrix properties. In the Clare Valley, South Australia, environmental tracer data are interpreted in conjunction with hydraulic data to constrain flow parameters in a fractured shale aquifer. Hydraulic conductivity, matrix porosity, fracture spacing, and groundwater age are measured, and a value for matrix diffusion coefficient is assumed. Equations describing tracer distribution and hydraulic properties of the system are solved simultaneously, to yield estimates of fracture aperture, vertical water velocity, and aquifer recharge rate. In particular, the recharge rate is estimated to be approximately 100 mm yr-1. A sensitivity analysis showed that this value is most sensitive to the measured values of matrix porosity and groundwater age, and highly insensitive to the measured hydraulic conductivity and the assumed matrix diffusion coefficient. A major horizontal fracture at 37 m depth intercepts most of the vertical flow. The leakage rate to the deeper flow system is estimated to be less than 0.1 mm yr-1.

This paper presents an autobiographical and biographical historical account of the genesis, evolution and resolution of the Elder Problem. It begins with John W. Elder and his autobiographical story leading to his groundbreaking work on natural convection at Cambridge in the 1960's. His seminal work published in the Journal of Fluid Mechanics in 1967 became the basis for the modern benchmark of variable density flow simulators that we know today as "The Elder Problem". There have been well known and major challenges with the Elder Problem model benchmark-notably the multiple solutions that were ultimately uncovered using different numerical models. Most recently, it has been shown that the multiple solutions are indeed physically realistic bifurcation solutions to the Elder Problem and not numerically spurious artefacts. The quandary of the Elder Problem has now been solved-a major scientific breakthrough for fluid mechanics and for numerical modelling. This paper-records, reflections, reminiscences, stories and anecdotes-is an historical autobiographical and biographical memoir. It is the personal story of the Elder Problem told by some of the key scientists who established and solved the Elder Problem. 2017 marks the 50 year anniversary of the classical work by John W. Elder published in Journal of Fluid Mechanics in 1967. This set the stage for this scientific story over some five decades. This paper is a celebration and commemoration of the life and times of JohnW. Elder, the problem named in his honour, and some of the key scientists who worked on, and ultimately solved, it.

A special session titled 'Complexity in Modeling: How Much is Too Much? was convened at the 2011 Geological Society of America Annual Meeting. The session had a variety of thought provoking presentations-ranging from philosophy to cost benefit analyses-and provided some areas of broad agreement that were not evident in discussions of the topic in 1998. Complexity will not go away simply by fiat; too many problems require complexity to adequately address societal needs and expectations. Fred Molz expanded the philosophical underpinnings of model complexity with an example of 'computer-aided thinking.' This term suggests a utility for models even if mathematical chaos violates the concept of a single unique reality or where a premise of classic model prediction fails. Daniel Abrams compared end-extremes of the complexity simplicity scale, where insights gained from intensive particle tracking from a 3-D groundwater flow model were also obtained using a simple exponential solution for predictions of watershed scale transit time distributions.

Finding and utilizing reliable water supplies for desert communities is important but difficult and expensive. This is especially the case in the remote north and northwest of South Australia, where there are a number of small Aboriginal communities with populations of less than a hundred people. Rainfall is sparse and intermittent (less than 200 mm/yr), and residents rely on groundwater for nonpotable supplies. Previous drilling in shallow sediments aiming for porous aquifers has led to problems with long-term well production. More recent wells targeting deeper fractured rock aquifers are showing more promise; unfortunately, such boreholes are more expensive and difficult to install. The aim of our work is to use natural-source magnetotelluric (MT) imaging to try and identify better sites for bores, and therefore reduce the risk of drilling a dry well. We hope to do this by imaging the 3D conductivity structure, allowing the identification of possible aquifers, and also by measuring the anisotropy due to water-bearing fractures, through the sensitivity of natural-source MT responses to electrically anisotropic layers in the subsurface. The first phase of our work, described here, is to test the method at a site where the hydrogeology is relatively well understood. The second phase will apply it to a community in need.

SUTRA (Saturated-Unsaturated TRAnsport) is a software package that simulates groundwater movement and solute or energy transport in subsurface environments. The program uses a two-dimensional hybrid finite-element and integrated finite-difference method, and fundamental to this numerical modelling is the issue of mesh generation. In order for the FEMCAD mesh to be compatible with the requirements of SUTRA, a complete interface between FEMCAD and SUTRA has been developed. (from Authors)

The two-dimensional finite-element model SUTRA was used to simulate density-dependent saline groundwater flow in a semi-confined aquifer and was employed to investigate the stream-aquifer interaction near the River Murray and Slaney and Punkah Creeks on the Chowilla floodplain before and after a major flood. Evapotranspiration and recharge during a flood have been incorporated into the numerical model. Simulation of a proposed salinity-mitigation scheme has been modelled in cross-section by incorporating various management options. The peak salt load to the creeks occurs immediately after a flood recedes and subsequently declines over a six-month period to the baseline salt load. The effect of pumping on the floodplain has also been simulated and, in the case of the proposed interception scheme, a drawdown of 0.2 m at the outer edges of the floodplain to 1.5 m at well sites is predicted. -from Authors

SUTRA (Saturated-Unsaturated TRAnsport) is a software package that simulates groundwater movement and solute or energy transport in subsurface environments. The program uses a two-dimensional hybrid finite-element and integrated finite-difference method, and fundamental to this numerical modelling is the issue of mesh generation. Present mesh construction provided by SUTRA is limited to rectangular, equally spaced finite elements in cartesian or radial-cylindrical coordinate systems. Mesh generation is often time consuming, and in many situations a much more versatile mesh is required for effective simulation. This is particularly true when modelling pumping wells, non-uniform recharge and discharge, non-homogeneous soil properties (such as hydraulic conductivity), and various boundary conditions. FEMCAD, originally developed to generate meshes for geomechanics problems, allows detailed and often quite complicated meshes to be constructed in a computer-aided design environment. -from Authors

Free convection is a process by which mixing of fluids occurs over larger scales and at faster rates than Fickian transport; this process is of critical importance for a range of environmental issues. Despite decades of research, the understanding of this phenomenon still largely stems from theoretical studies, lab experiments, numerical modeling, and secondary field observations. In this contribution, we present a time-lapse field study of free convection in a coastal sabkha in the United Arab Emirates. Using electrical resistivity imaging data collected during three field campaigns, we demonstrate that free convection at this site originates from episodic large rainfall events that drive salt dissolution at the surface and infiltration of a dense brine to the water table. Our observations, including the rates of downward migration and dissipation of the dense fingers, are corroborated by hydrodynamic modeling of the system.

Baseflow estimation is an important problem in both low flow and flood hydrology and is commonly carried out using recursive filters. However, this approach is based solely on the observed streamflow hydrograph without consideration of physical catchment characteristics, such as soil properties, topography, vegetation and climate, etc. This paper investigates the performance of the commonly used Lyne and Hollick filter in baseflow separation for catchments with sandy soils with different properties. This is done by comparing the baseflow hydrographs obtained using the Lyne and Hollick filter with simulated baseflow from a 3D hypothetical catchment using the fully-integrated HydroGeoSphere model. In addition, the sensitivity of filter performance to different values of the Lyne and Hollick filter parameter is investigated. The results obtained indicate that the performance of the Lyne and Hollick filter varies significantly with soil characteristics and can be improved considerably by adjusting the Lyne and Hollick filter parameter based on soil characteristics.

In Australia, native vegetation clearance has had considerable impacts on surface water and groundwater salinities. The impact on surface water-groundwater connectivity is less understood. Ahydraulic, hydrochemical, and tracer-based study was conducted at two contrasting fractured rock catchments in South Australia. Results indicate that connectivity was variable across each of the catchments. The influence of the fractured rock aquifer was minimal in the pristine, uncleared Rocky River catchment, whereas in the cleared, mixed land-use Cox Creek catchment, the fractured rock aquifer played a more significant role. The results emphasise the need to understand the importance that the impacts of land-use change (particularly vegetation clearance) can have on surface water-groundwater connectivity. © 2010 Taylor & Francis Group, London.

The aim of the research program, funded by the Geoscientists Without Borders?program, is to undertake geophysical surveys to locate groundwater resources for Aboriginal communities in remote and arid parts of South Australia. These communities currently rely on boreholes in deep-aquifers (> 50 m to the water table) within fractured rock for non-potable water supplies. Maintaining and developing existing groundwater supplies and identifying new resources are essential. In mid-2010 we will collect magnetotelluric (MT) data across an array of sites to image the three-dimensional (3D) electrical conductivity structure from the surface to a depth of 1 km or greater, and the anisotropy of electrical conductivity due to fracture alignment. Variability in conductivity both laterally, and with depth, will yield a better method of identifying groundwater resources to target boreholes, and hence reduce the number of dry wells. The research program will have three principal outcomes: 1. To determine optimal borehole locations for the Aboriginal communities to develop sustainable groundwater resources; 2. To better understand and make recommendations as to how MT instruments can be deployed to reduce the risk of drilling dry holes; and 3. To more fully understand how groundwater in fractured rock can be effectively imaged by electrical conductivity variability.

Salinization of water resources is a major problem on a global scale. In the Loxton-Bookpurnong irrigation area (South Australia) saline groundwater is pushed towards the river by advective flow. Groundwater was sampled from beneath the river. In all cases, fresh river water overlies saline groundwater, a stable stratification with regard to the fluid density. The salinities encountered partly exceed those of seawater. Modelling was done to determine what role variable-density flow behaviour plays in surface water-groundwater interaction in a stably-stratified fresh surface water-saline groundwater interface. Comparison of measured and simulated concentrations vs depth profiles reveals a qualitative resemblance of the profiles for simulations where density was the dominating force. The clear stratification of fresh over saline water, with a sharp increase at a certain depth, leads to the hypothesis that the high density of the fluid may partly suppress discharge in stagnant pockets below the river. Copyright © 2008 IAHS Press.

The movement of salt from a saline lake bed/evaporation basin to the underlying groundwater system is investigated. The density dependent flow behaviour has been modelled in cross section using a 2-D finite element model SUTRA. Due to the inherent salinity contrast between the saline lake brine and groundwater, the system is characterised by a state of density stratification which is the major cause of the resulting instability. Mixed convection flow (hydraulically and buoyancy-driven) occurs which subsequently controls the solute concentrations below the lake bed in order to achieve a stable density gradient (Rayleigh convection). This type of flow regime is particularly relevant to saline disposal basins in the Murray Darling Basin of Australia. Numerical examples simulating 2-D brine convection from lake bed salt, taking moderate 102 to high 105 Rayleigh numbers are given. The conditions which define the onset of such convective phenomena are investigated. Rayleigh number analysis of the convective flow is employed and is seen to be an extremely powerful tool for predicting the long term behaviour of saline evaporation basins.